What is the Manually Set checkbox

It is the little blue triangle in the upper right hand corner of a cell. To clear it, just hover above the cell, it will give you a prompt, then click the "here" hyperlink.

But if i click "here" my duration decreasing to one day

For scheduling, the best practice is to set duration and create dependencies, and then let the system calculate start date and due date based on this relationships OR to set a fixed work policy and set work, units, and dependencies, and let the system calculate the duration and dates for you.  I would recommend hopping on a Q&A forum to review this with a customer success manager.